IBut friction drive is a seriously sucky design. It creates a lot of tire wear, and needs a lot of pressure between roller and tire. Plenty of losses, and unless properly designed, the bearings don't last long.
The trouble with small combustion engines is economy of scale. A small one takes the same number of parts. And surface area vs volume etc makes it harder to get a good burn out of them.
So plenty of reasons to go big(ger). More power, better burn at very close to the same engineering cost.
But sure, a good combustion-assist engine for bikes could have its uses. Maybe a fuel cell?